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Myriora Maths › Maths foundations › Chapter 13: First steps in algebra

The missing number

Ages 5–7Chapter 13 · Lesson 1

In this lesson

Your turn

Something times four makes twenty-eight. What's in the box?

Show the answer

Twenty-eight divided by four is seven. Check: seven times four is twenty-eight!

The common mistake

Here's a question that catches lots of people. Something take away four makes ten. What's in the box? Some people say six, because ten take away four is six. But six take away four is only two! Work backwards the other way: ten add four is fourteen.

Try it at home

Write box sums for each other (□ + 7 = 15, □ × 3 = 24) and check each answer by putting it back in.

Read the whole lesson

Hello, and welcome to Myriora Maths! Here's a sum with a number hiding in a box. Something add five makes twelve. What's in the box? Let's find out!

The box is hiding a number. We don't know it yet. But we know the box add five makes twelve.

Think: what add five makes twelve? Count on from five to twelve: six, seven, eight, nine, ten, eleven, twelve. That's seven jumps. Or work backwards: twelve take away five is seven.

Always check. Put seven in the box. Seven add five is twelve. It works!

Boxes can hide in other sums too. Something times three is fifteen. Fifteen divided by three is five. Twenty take away something is fourteen. Twenty take away six is fourteen, so it's six.

Let's try one together. Something add eight makes fifteen. What's in the box? Say it out loud!

Fifteen take away eight is seven. Check: seven add eight is fifteen!

Here's a question that catches lots of people. Something take away four makes ten. What's in the box? Some people say six, because ten take away four is six. But six take away four is only two! Work backwards the other way: ten add four is fourteen.

Your turn! Something times four makes twenty-eight. What's in the box? Pick the right card. You've got twenty seconds, or pause the video. Go!

Time's up! Twenty-eight divided by four is seven. Check: seven times four is twenty-eight!

Quick review. A box can hide a number in a sum. Work backwards to find it: take away undoes add, and divide undoes times. Then put your answer in the box, and check. See you next time!